This invention relates to automobile air-conditioning system, and in particular to temperature control circuits in those systems.
It is well known in the prior art that the automobile air-conditioning system comprises a refrigerant system including a refrigerant compressor and a refrigerant evaporator, a temperature control circuit for controlling the operation of the refrigerant compressor in response to the temperature detected, and a blower system for blowing the cooled air into the room of the automobile. The refrigerant compressor is generally driven by the automobile engine, and an electromagnetic clutch is used for selectively transmitting the engine output to the refrigerant compressor. The temperature control is made by controlling operation of the electromagnetic clutch.
In a known temperature control, a temperature detecting element or a temperature sensor such as a thermistor is mounted on the refrigerant evaporator or in an air duct in which the cooled air flows to be blown into the automobile room. When the temperature detected by the temperature sensor is lowered than a predetermined temperature, the electromagnetic clutch is deenergized so that the compressor is not driven. When the temperature elevates to another predetermined higher temperature, the electromagnetic clutch is energized to transmit the engine output to the compressor so that the temperature is again lowered. Thereafter, similar operations are repeated so that the temperature in the automobile room may be maintained constant.
Generally speaking, there is provided a temperature difference between a temperature (this will be referred to a "compressor-stopped temperature") at which operation of the compressor is stopped and another temperature (this will be referred to a "compressor redriven temperature") at which the compressor is again driven. Considering that the frost on the evaporator degrades the cooling effect, the temperature difference must be determined large to prevent the evaporator from being frosted. This means that a time period when the compressor is not driven is long, and the temperature in the room, therefore, is not maintained comfortable. That is, when the compressor-stopped temperature is determined lower, the room is cooled excessively and, on the contrary, when the compressor-redriven temperature is determined high, the room is not cooled sufficiently.
In another known temperature control, the temperature sensor is disposed in the room. Because the temperature in the room is not always corresponding to the temperature of the air blown from the system into the room, and because the temperature variation of the room considerably delays from the variation of the temperature of the blown air, the temperature difference between the compressor-stopped temperature and the compressor-redriven temperature must be determined very small. Even if the small difference is determined, the temperature in the room does not elevate to the determined compressor-redriven temperature and the temperature of the blown air is sometimes elevated to make the persons uncomfortable.